refactor: move act under internal

The runner is an application, not a library. `act/model` and
`act/exprparser` were the last packages anything outside this repository
consumed and they now live in actionslib, so nothing needs the rest of
`act` to be importable, and keeping it importable invites the coupling
that was just removed.

Import paths only, the files are unchanged.

Assisted-by: Codet:GPT-5.1-Codex

internal/act/runner/testdata/evalenv/push.yml

@@ -0,0 +1,21 @@
+name: CI
+
+on: push
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    env:
+      BUILD_DIR: ${{ github.workspace }}/build
+    steps:
+      - name: Dump
+        run: |
+          echo "$BUILD_DIR"
+          echo "$EXPECTED_BUILD_DIR"
+          echo "$GITHUB_WORKSPACE/build"
+        env:
+          EXPECTED_BUILD_DIR: ${{ github.workspace }}/build
+      - name: Test
+        run: |
+          [ "$BUILD_DIR" = "$EXPECTED_BUILD_DIR" ] && [ "$BUILD_DIR" = "$GITHUB_WORKSPACE/build" ]
+        env:
+          EXPECTED_BUILD_DIR: ${{ github.workspace }}/build